index.twig

{% for key, val in data %}
    Key: {{ key }}
    Value: {{ val ?: "0" }}
{% endfor %}

lazy_twig.php

<?php

require_once('vendor/autoload.php');

$twig = new Twig_Environment(new Twig_Loader_Filesystem('.'), ['debug' => true]);

class LazyContext implements ArrayAccess, IteratorAggregate
{
    private $storage = [];
    private $generator;

    public function __construct(Generator $generator)
    {
        $this->generator = $generator;
    }

    private function waitForMore()
    {
        // NB: The try-finally construct is weird, but necessary for the fastest speeds!
        //     We want to yield before advancing the generator, to avoid waiting for the next item
        //     However, this means that if waitForMore is terminated early, it never advances
        //     This causes the last key to be emitted twice

        //     The sane but less-speedy way is to move the yield after the next, drop the try-finally
        try {
            // Foreach resets the generator on each call to waitForMore, so we'll iterate by hand
            while ($this->generator->valid()) {
                $key = $this->generator->key();
                $value = $this->generator->current();

                $this->storage[$key] = $value;
                yield $key;

                $this->generator->next();
            }
        } finally {
            if ($this->generator->valid()) {
                $this->generator->next();
            }
        }
    }

    public function offsetGet($offset)
    {
        if (array_key_exists($offset, $this->storage)) {
            return $this->storage[$offset];
        }

        foreach ($this->waitForMore() as $key) {
            if ($key === $offset) {
                return $this->storage[$offset];
            }
        }

        throw new OutOfBoundsException("Key {$offset} is not set");
    }

    public function offsetExists($offset)
    {
        if (array_key_exists($offset, $this->storage)) {
            return true;
        }

        foreach ($this->waitForMore() as $key) {
            if ($key === $offset) {
                return true;
            }
        }

        return false;
    }

    public function offsetSet($offset, $value)
    {
        throw new LogicException('Context is read-only');
    }

    public function offsetUnset($offset)
    {
        throw new LogicException('Context is read-only');
    }

    public function getIterator()
    {
        foreach ($this->storage as $key => $value) {
            yield $key => $value;
        }
        foreach ($this->waitForMore() as $key) {
            yield $key => $this->storage[$key];
        }
    }
}

function controllerFunc()
{
    yield 'template' => 'index.twig';

    foreach (range('a', 'z') as $key) {
        $shouldWait = (bool)random_int(0, 1);
        if ($shouldWait) {
            sleep(random_int(1, 3));
        }
        yield $key => $shouldWait;
    }
}

$ctx = new LazyContext(controllerFunc());
echo "Template is: {$ctx['template']}\n";
$twig->display($ctx['template'], ['data' => $ctx]);